Flame fusion crystal growing employing vertically displaceable pedestal responsive to temperature



May 27, 1969 KENICHI SHIROKI ET AL 3,446,602

FLAME FUSION CRYSTAL GROWING EMPLOYING VERTICALLY DISPLACEABLE PEDESTAL RESPONSIVE T0 TEMPERATURE Filed NOV. 9. 1966 aeawwa cnysnu rap LEVEL r: {WEEHOGOVHE 451/51.

W5 5 m m a 5 I L H m 7 U S 4 A M P G M M n H" a v. I w 5 r A L 6,. m ar m M4 Q m? I ww wmw 0 a s w-mk 3Q 35355 .QQE wwumtwk United States Patent 3,446,602 FLAME FUSION CRYSTAL GROWING EMPLOYING VERTICALLY DISPLACEABLE PEDESTAL RE- SPONSIVE TO TEMPERATURE Kenichi Shiroki and Seiichi Saito, Tokyo, Japan, assignors to Nippon Electric Company, Limited, Tokyo, Japan, a corporation of Japan Filed Nov. 9, 1966, Ser. No. 593,029 Claims priority, application Japan, Nov. 13, 1965, 40/ 69,672 Int. Cl. B01d 9/00 US. 'Cl. 23-273 2 Claims This invention relates generally to flame fusion crystal growing techniques, and in particular to a novel flame fusion type crystal growing apparatus which insures homogeneous crystal growth.

It is common practice, when growing crystals utilizing flame fusion techniques, to drop fine particle size feed material into an oxyhydrogen or plasma jet flame, thereby causing the material to melt and accumulate on a refractory pedestal. The technique is common with or without placing a seed crystal on the pedestal beforehand.

To insure steady growth when employing the above technique, the pedestal is gradually lowered either manually or semiautomatically by observing the growing boule top through a peep hole in the side wall of the muflle.

It has been found that the flame surrounding the growing crystal has a steep thermal gradient with a mean value as high as 100 C./cm. for an oxyhydrogen flame.

Since the temperature distribution varies considerably with changes in the level of the growing boule top, a major cause of the inhomogeneity in the concentration of the crystal constituents is effected, as is thermal stresses in the grown crystals resulting in nonuniform properties of the finished product.

To overcome the foregoing problems, the growing boule top must be maintained at as constant a level as is possible in the furnace.

The eye-observation practice previously mentioned does not present a completely satisfactory solution to this requirement in that observation inaccuracies occur due to eye fatigue and normal variations between people, rendering the products reliability uncertain.

It has been recognized, therefore, that better arrangements must be afforded to the growing of crystals to insure that the top is maintained as constant a level and temperature as possible. A conventional method employs the intense light beam emitted from the growing crystal top (which is heated to an extremely high temperature). The light beam is guided through a suitable light path onto a photosensitive element such as a phototube. When the growing crystal top deviates from the desired crystal level, the light beam fails to impinge on the photosensitive element, and the detected output of the latter decreases. In response to a lowered output, a motor vertically displaces the pedestal through a level regulator, thereby maintaining the crystal top at substantially a constant level.

The foregoing arrangement, however, is still not completely satisfactory in that due to the accumulation of the fed powder, the light pathis contaminated; the light further flickering due to the presence of combustion gases, etc., rendering the arrangement, while more reliable than that previously discussed, still unsatisfactory.

Accordingly, it is the object of this invention to provide a flame fusion crystal growing apparatus of the automatic type which is simple in construction, inexpensive to manufacture and operate, and which is free from the defects previously mentioned and is capable of maintaining the growing crystal top automatically at the desired level.

3,446,602 Patented May 27, 1969 Briefly, the invention is predicated upon the recognition of the fact that the inside mufile wall temperature near the growing crystal top is predominantly governed by the heat radiated from the white-hot top rather than the flame itself. In other words, the heat radiation from the flame contributes much less to the muffle wall temperature than that from the incandescent crystal top.

From the recognition of this fact, the invention constitutes a temperature sensitive element imbedded in the muflle which is responsive to changes in the muflle temperature caused by the deviation of the boule top from the desired level to suitably displace the crystal mounting pedestal vertically.

The above mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will best be understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings wherein:

FIG. 1 schematically illustrates an automatic flame fusion crystal growing apparatus according to the invention; and

FIG. 2 graphically depicts the muflle temperature vs. boule top level characteristic of the apparatus shown in FIG. 1.

Turning now to the invention, and in particular to FIG. 1, there may be seen a pedestal 12 of refractory material upon which is seated a growing single crystal (for example, of ruby) 11. A muffle 17 embraces the upper portion of pedestal 12 and the entire crystal 11. A thermocouple 15, for exam le of the Pt-Pt/Rh type, is imbedded in the mufile at a level slightly higher than that at which the growing boule top is to be maintained. The developed in the thermocouple is fed to a level regulator 16 for causing motor 14 to either initiate or discontinue in response thereto. Transmission mechanism 13 may be of any of the types known to the art for translating the rotary motion of the motor to rectilinear motion of the pedestal 12. For simplicity, FIG. 1 has omitted details of the muflle and the remaining portions of the flame fusion furnace and burners well known to the art.

The growing crystal top is heated by a flame (not shown) to incandescence and emits radiated heat. Since, as mentioned, the muflle wall surrounding the crystal top is affected much more by the local radiation of the incandescent crystal than from the flame itself, the thermoelectromotive force developed in the thermocouple increases or decreases as the growing crystal top moves towards or away from the thermocouple level; there being a direct tendency or a one-to-one correspondence between the temperature indication or and the growing crystal top level.

FIG. 2 illustrates an exemplary relationship of the thermocouple temperature indication and the growing crystal top level (in cm.) for a ruby crystal growth. It may be assumed that the thermocouple temperature indication is proportional to the thermoelectromotive force developed in the thermocouple. In this graph, A and B represent respectively the desired growing crystal top level and the corresponding temperature indication of the thermocouple.

In order to maintain the growing crystal level at point A, it is only necessary to set the level regulator in such a manner that when the thermoelectromotive force developed by the thermocouple exceeds the value at point B, the motor responds to lower the pedestal, and when the growing crystal top level is less than the desired level A and accordingly, the developed in the thermocouple is less than the value at point B, the level regulator causes the motor to discontinue operation until the crystal top (which keeps on growing upward) recovers to the desired level A. In other words, the level regulator may simply discriminate whether the output of the thermocouple exceeds or not a predetermined value, operating the motor in accordance therewith. Since such sensing arrangements are well known to the art, the circuit details will not be analyzed here.

The start-stop motor operation is performed repeatedly during the crystal growth process to maintain the molten top of the growing crystal at substantially the predetermined level.

Extensive use of the inventive arrangement has been found to produce rapid and smooth recovery to the desired level of the growing crystal top, producing a crystal homogeneity markedly superior to that effected by any of the known conventional methods.

While the principles of this invention have been described in conjunction with a specific embodiment for preparing a single crystal of ruby, it will be appreciated by one skilled in the art that suitable modifications can be made without substantially departing from the spirit of this invention as set forth in the accompanying claims.

What is claimed is:

1. In a crystal growing apparatus of the flame fusion type employing a mufile embracing a vertically displaceable pedestal for seating the crystal to be grown, and means feeding molten material on the said seated crystal,

the improvement comprising: a temperature sensitive element imbedded in the mufile wall proximate the level at which the growing crystal top is to be maintained; means for vertically displacing said pedestal within said mufile; and means coupled between said displacing means and said temperature sensitive element for controlling the operation of said vertical displacement means.

2. The improvement claimed in claim 1 wherein said vertical displacement controlling means is responsive to a predetermined level in the output of said temperature sensitive element for initiating or discontinuing the operation of said vertical displacement means.

References Cited UNITED STATES PATENTS 3,190,728 6/ 1965 Vunderink. 3,265,470 8/ 1966 Keller. 3,314,769 4/ 1967 Rudness et al.

NORMAN YUDKOFF, Primary Examiner.

V. W. PRETKA, Assistant Examiner.

US. Cl. X.R. 23-301 

1. IN A CRYSTAL GROWING APPARATUS OF TH FLAME FUSION TYPE EMPLOYING A MUFFLE EMBRACING A VERTICALLY DISPLACEABLE PEDESTAL FOR SEATING THE CRYSTAL TO BE GROWN, AND MEANS FEEDING MOLTEN MATERIAL ON THE SAID SEATED CRYSTAL, THE IMPROVEMENT COMPRISING: A TEMPERATURE SENSITIVE ELEMENT IMBEDDED IN THE MUFFLE WALL PROXIMATE THE LEVEL AT WHICH THE GROWING CRYSTAL TOP IS TO BE MAINTAINED; MEANS FOR VERTICALLY DISPLACING SAID PEDESTAL WITHIN SAID MUFFLE; AND MEANS COUPLED BETWEEN SAID DISPLACING MEANS AND SAID TEMPERATURE SENSITIVE ELEMENT FOR CONTROLLING THE OPERATION OF SAID VERTICAL DISPLACEMENT MEANS. 